The more sex partners young women have, the more likely they are to become dependent on drugs and alcohol, says a new study.

University of Otago researchers found young people in general were at greater odds of developing alcoholism and cannabis dependency the more sex partners they had - with the odds even greater for young women.

The study tracked the health and behaviour of more than 1000 people from birth in Dunedin in 1972-73.

Researchers looked at how many sex partners the study participants had during three age periods: 18-20, 21-25, and 26-31 years.

They then looked at their mental health immediately after each period, including anxiety, depression, and substance dependence.

The study's lead author, Dr Sandhya Ramrakha, said women who had 2.5 or more partners each year increased their odds of having a substance dependence disorder by up to 17 times.

"This is a striking increase in the risk of substance disorder," said Ramrakha.

"Furthermore, when we used a model to compare men and women who had more than 10-20 sex partners in the same periods, we found that these women were much more likely to have a substance disorder than the men."

She said the strong link between the number of sex partners and substance disorders stayed even after taking into account pre-existing mental disorders and substance problems.

The researchers said an explanation for the results could be that sex, drugs, and alcohol were part of a cluster of "risk-taking behaviours" that developed in adolescence.

"Or it may be due to the disinhibitory effects of alcohol and cannabis providing opportunities for sexual behaviour," said Ramrakha.

"Context may be another explanation, that is, pubs and bars are also places where one can easily meet partners."

Ramrakha said more research was needed to figure out what was behind the link so solutions could be found.